One type of an impairment in a network is robbed bit signaling (RBS) which is a technique used in telephone networks to perform various control functions. RBS involves modifying data transmitted over a network, in particular, the least significant bit (LSB) of certain octets being transmitted. Though RBS is generally acceptable when the octets being transmitted in the network are used to carry voice or voice-data signals, with pulse code modulation (PCM) modems it effectively acts as noise or distortion which may cause high data error rates.
Many modern digital networks which may carry PCM modem data are constructed as T-carrier systems that use robbed bit signaling. The digital data transmitted over these networks is grouped into octets (eight (8) bits) and the octets are grouped into frames (twenty four (24) octets). In FIG. 1 there is shown a frame 10 containing 24 octets, O.sub.1 -O.sub.24. The frames transmitted over the network are continuous, and the single 24 octet frame 10 is shown for descriptive convenience only. As indicated above, some of the octets are affected by RBS. That is, the network uses the least significant bit (LSB) position of the affected octets to carry data to perform control functions in the network. Thus, for example, the first octet, O.sub.1, may affected by a type of RBS that forces the LSB of that octet to one, as indicated by the "F" in that octet. Depending on the octet of data carried in that interval, RBS may change that octet's data. In particular, if that octet had a zero in its LSB, RBS alters that octet. If, however, that octet had a one in its LSB, RBS would have no affect on the octet from the end-user's perspective.
It has been observed that RBS has deterministic periodicity with the most basic period being six octets. Therefore, in this example, the next affected octet is octet O.sub.7, which is six octets removed, and so on. (The designation "NC" means "no change"). Since RBS recurs every six octets, the octets can be viewed as appearing in a basic period 12 of six time slots or intervals, 1-6, which may or may not be affected by RBS. For example, octets O.sub.1, O.sub.7, O.sub.13 . . . etc., appear in interval "1" which is affected by RBS, while octets O.sub.2, O.sub.8, O.sub.14 . . . etc., appear in interval "2" which is unaffected by RBS. It should be noted that due to the nature of the networks, it is possible to have more than one RBS affected interval in the basic period of six.
Another type of impairment in digital networks is a digital loss (typically 3 or 6 dB) in the network forced by digital pads. This loss affects all octets transmitted through the network. For example, this loss can cause a bit forced from a zero to a one by RBS to appear to be a zero. Thus, In certain network RBS/digital pad arrangements, such as when digital loss due to digital pads occurs before RBS and when there is RBS and digital loss followed by RBS, RBS may be difficult to detect and compensate for.
Therefore, a need exists for detecting and characterizing RBS and RBS/digital pad arrangements in networks in order to provide PCM modems information about these network impairments so that modem performance can be improved.